Sensor size vs. depth of field?

[svenning]

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Dear Leica Forum members,

 

Why does a digital camera with a small sensor have a much greater depth of field than a Full Frame digital camera?

 

To check it out I made the following scenario:

I photographed a green glass vase with the use of two cameras.

 

Firstly I mounted a Leitz 180mm APO-Telyt lens on a Canon EOS 5DMK2 digital camera with a full-frame sensor (36x24mm). Aperture F/4.0 and a focal distance of about 2 meters.

 

Secondly I used a Panasonic Lumix DMC-FZ50 with a 1/1.8 " (7.18 x 5.32 mm) sensor and made a focal lenght setting of 38mm (which equals 180mm in the 35mm film). Aperture F/4.0 and a focal distance of about 2 meters.

 

The difference between the two pictures showed that the smaller sensor had much greater depth of field than the Full Frame sensor.

 

My question is now: Why the large difference in the depth of field?

 

Your reply is highly appreciated - Best regards Svenning, Denmark.

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[SJP]

There are substantial numbers of threads on this topic, seach for crop factor, depth of field (DoF) etc. but to cut a long story short:

 

If you have a cropped sensor the effective (35mm full-frame equivalent) focal length is the lens focal length time the crop factor (1.33 for a M8, more than 4 for your Panasonic Lumix DMC-FZ50). This part is obvious.

 

What is less obvious is that the factor that governs the Depth-of-Field, the socalled circle-of-confusion, also changes by that same factor so instead of 30 micron for FF, it becomes about 8 micron (30/4). This is because you need to enlarge the small sensor image more to get the same size file/print. If you then calculate the depth of field of this situation you end up with the rough rule of thumb that the equivalent aparture is increased by the crop factor. So your Lumix lens at f/2.8 will "look" like a f/11 lens as far as the depth of field is concerned. Exactly as you see with your pictures, which actually is a very nice example of this effect.

 

More on this here, if you want to know the details (p13-14 for equivalent aparture). Calculate yourself via this spreadsheet.

[jaapv]

Just to add, you start of by "digital camera"and "sensor"This effect has nothing to do with that. Medium Format cameras will produce much shallower DOF than a 135 camera on film as well..

[svenning]

Dear Dutchmen,

 

Thank you for your answers - that helped alot for my understanding of the problem.

 

I believe that the larger sensor of the new Leica S2 will make focussing even more critical at full open apertures?

 

Again thank you for your answers.

 

All the best - Svenning, Denmark.

[Alberti]

Dear Dutchmen,

 

Thank you for your answers - that helped alot for my understanding of the problem.

 

I believe that the larger sensor of the new Leica S2 will make focussing even more critical at full open apertures?

 

Again thank you for your answers.

 

All the best - Svenning, Denmark.

 

Right, spot on.

While the M-system rangefinder has the best focussing mechanism from any camera and can cope with a F1.0 opening, the S2 would -with it's larger sensor- have to out-do it, but on the other hand, the S2 is packaged with less large F2.5 lenses, so the strain on the mechanism won't be unsurmountable.

 

Alberti

[Shootist]

The simple answer is on small sensor cameras you use a smaller, less/lower mm, lens to capture the same size image as you would using a 35mm size sensor. As the lens mm go down you get greater DoF.

So if you used the panasonic at 38mm and then a FF sensor camera with 38mm lens the DoF would be the same.

[01af]

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When shooting with different format cameras from the same distance at the same angle-of-view (i. e. equivalent focal length) and same aperture then for the smaller format, the shorter focal length will increase, the smaller circle-of-confusion will decrease depth-of-field. But the two factors don't cancel each other out. Instead, the COC's diameter goes into the DOF formula once; the focal length goes into it squared (i. e. twice). That's why the increase with shorter focal length outweighs the decrease with COC.

 

When using the same (as opposed to equivalent) focal length on different format cameras from the same distance and at the same aperture then the smaller format will yield less DOF due to the smaller COC. However you'd also get a different image, with a narrower angle-of-view and higher magnification.

[Jeff S]

Enters one more time to say...

 

Welcome to ReidReviews

 

Great article on small sensor cameras...and lots of other essays...plus excellent equipment reviews. The small subscription price will be earned back in value in no time.

 

Exits...

 

Jeff